Ubirajara rivulus (killifish)

Where they come from

Ubirajara rivulus (often listed as Melanorivulus ubirajarai) is one of those little South American rivulus killies that comes from small streams, seepages, and shallow margins where plants and leaf litter do most of the work. Think warm water, gentle flow, lots of cover, and micro-food everywhere.

They are not the "puddle that dries out tomorrow" type. They are more of a small-creek/edge-of-stream fish, so you can set them up like a calm, planted nano and they look right at home.

Setting up their tank

If you give them a mature, planted tank and a tight lid, you are already 80% there. These fish are alert, fast, and absolutely willing to test any gap in the cover.

• Tank size: 10 gallons is comfortable for a pair or trio. You can do smaller, but stability gets touchier and they spook more easily.
• Lid: non-negotiable. Cover filter cutouts too. I have had rivulus launch through surprisingly small openings.
• Filtration: sponge filter or a gentle HOB with the flow baffled. They do not appreciate being pinned in a current all day.
• Hardscape: leaf litter (catappa/oak), small wood, and lots of plants. Floating plants help them feel bold.
• Substrate: anything works, but a darker bottom calms them down and makes the colors pop.

Water parameters do not need to be mystical. Mine have done well in neutral to slightly acidic water. Aim for steady temps in the mid-70s F (around 24 C), and focus on clean, mature water rather than chasing numbers.

What to feed them

They are small predators with small mouths. Live and frozen foods get the best response, and you will see better color and more confident behavior.

• Staples I use: baby brine shrimp, daphnia, cyclops, grindal worms, microworms
• Frozen options: bloodworms (small ones), brine shrimp, daphnia
• Dry foods: they can learn to take micro pellets or crushed flakes, but do not be shocked if they ignore it at first

Watch their bellies. It is easy to overdo worms and bloodworms with killies. I like to rotate: one day BBS, next day daphnia, then something frozen, then back to BBS.

How they behave and who they get along with

They have that classic rivulus vibe: curious, a little spooky, and always watching you. Males can be pushy with each other in small tanks, but it is usually more chasing and flaring than real damage if the tank is planted.

• Best setup: one male with one or two females
• Multiple males: possible in a bigger, heavily planted tank, but expect hierarchy drama
• Tankmates: small, calm fish that will not outcompete them at feeding time
• Inverts: assume shrimp fry will be snacks. Adult shrimp sometimes coexist, but babies rarely make it

They also do the "freeze and bolt" thing if the room is loud or you come in too fast. Floating plants and a darker background make a big difference in how often they are out front.

Breeding tips

Breeding them is doable, but it is not like tossing in a mop and collecting a hundred eggs the next day (at least it never was for me). They are more of an "egg here and there" fish, and patience pays off.

• Spawning media: a thick clump of Java moss, a yarn mop, or fine-leaf plants like guppy grass
• Conditioning: live foods for 1-2 weeks and slightly warmer water helps get them going
• Egg collection: check moss/mop every few days, or just move the media to a small hatching container
• Hatching: clean water, gentle aeration, and keep fungus down by removing bad eggs

For fry, the first foods matter. I start with infusoria or vinegar eels if they are tiny, then move to baby brine shrimp as soon as they can take it. A mature, plant-packed grow-out tank also works great because there is always something alive to pick at.

Common problems to watch for

• Jumping: the #1 killer. Any gap in the lid is an exit.
• Skinny fish that "won't eat": usually stress, too much flow, or tankmates stealing food. Fix the setup first, then tempt with live foods.
• Fin nips and chasing: happens in bare tanks or tight quarters. Add cover, reduce males, or rearrange the scape.
• New fish crashing: they can come in touchy. Go slow on acclimation and keep the tank mature and stable.
• Internal parasites: if a fish eats but stays thin, consider quarantining and treating (common with wild-origin fish).

If you keep them warm, calm, well-fed, and covered, they are honestly pretty forgiving for an "intermediate" fish. Most issues I have seen came down to too much current, not enough cover, or the lid problem.